This invention relates to a process for producing a metal foil coated with flame sprayed ceramic with high productivity.
Flame spraying of ceramic is widely known as an effective technique for ceramic coating. Especially, ever since advent of this flame spraying technique, by taking advantage of its excellent mass productivity, it has been popularly used for the improvement of surface qualities such as wear resistance, heat resistance, surface hardness, electrical insulating properties, heat insulating properties, etc., of mostly metallic products.
However, use of this technique, namely flame spraying of ceramic, for forming a ceramic coat on a thin metal foil involves a serious problem. It originates in the principle of this technique according to which a flame spray coating material, or ceramic, is supplied into an ultra-high temperature atmosphere such as oxygen-acetylene gas combustion flames or plasma flames of argon gas, nitrogen gas, helium gas or the like to melt the coating material (ceramic) and the melt is hit against the surface of the object to be coated (metal foil), and then cooled and solidified.
Generally, when ceramic is flame sprayed to a metal foil under the conditions used for the ordinary objects to be coated such as metal plates, metal rolls and the like, the heat of ceramic which adhered in a molten state to the metal foil during flame spraying operation is accumulated in the metal foil to cause its oxidation or its fusion and partial break, making it unable to obtain a satisfactory product.
A conceivable measure for overcoming this problem is to provide air nozzles on both sides of spray gun and perform flame spraying while blowing cold air against the object surface. This method, however, is still unable to prevent discoloration or break of the metal foil in the course of flame spraying.
As a result of many and various studies on the subject matter, the present inventors were convinced that in order to obtain a ceramic flame sprayed metal foil free of defects by using the conventional techniques, there is no other effective means but to perform the flame spraying operations under the conditions which can minimize the influence of heat on the metal foil. This necessitates a reduction of output of the spray gun and to notably reduce the ceramic spray rate per unit time.
This method is indeed capable of producing an excellent metal foil coated with flame sprayed ceramic, but it has a serious drawback. That is, this method is excessively low in productivity because of reduced output of spray gun and very low ceramic spray rate per unit time which are inevitable for minimizing the influence of heat on the metal foil.
The spray rate of ceramic per unit time is in almost direct proportion to film forming rate of ceramic layer, so that an excessive lowering of spray rate leads to a marked reduction of productivity. Also, reduced output of spray gun lowers the temperature of flames for melting ceramic, which retards melting of ceramic. Therefore, even if the ceramic feed into spray gun is unchanged, there may take place imperfect melting of ceramic if the output of spray gun is low, and also the molten ceramic becomes hard to adhere to the object because of low temperature, resulting in a low coating efficiency.
Due to these problems, productivity was very low in forming a ceramic coat on metal foils by flame spraying of ceramic with the conventional techniques, and such flame spray coating on metal foils would take 10 to 20 times as much time as required for flame spray coating to an object with a large thickness such as metal plates. Thus, mass production of metal foils coated with flame sprayed ceramic has been quite impossible in the prior art.
On the other hand, U.S. Pat. No. 4,713,284 discloses a process for producing a ceramic coated laminate wherein flame spraying of a ceramic powder is conducted on a copper foil running on a cooling roll in which cooling water is passed. This process has a disadvantage in that discoloration takes place probably due to loss of heat by the roll-constituting material and insufficient cooling capacity.